Optical fiber production generally involves drawing a glass fiber from a glass preform, which usually is of silica glass, then applying a layer of coating material to the fiber. The coating is applied instantly after draw to prevent contamination or contact of any kind with the nascent fiber surface. The coating material is typically a UV curable polymer. Dual coated substrates, in particular optical fibers, are usually coated with a first layer of relatively soft polymer and a second layer of a higher modulus polymer for maintaining high strength and abrasion resistance. The coated fibers must be capable of withstanding, over their entire length, a maximum stress level to which the fiber will be exposed during installation and service. A single fiber failure can result in the loss of several hundred circuits.
Optical fibers are usually coated by a wet coating process which typically involves passing the newly drawn fiber through a reservoir of liquid prepolymer material and then curing the prepolymer by exposure to curing radiation, most commonly, ultra-violet light. In the dual coating process, coatings are applied in tandem or simultaneously (within the same applicator or die assembly). The tandem arrangement applies a first coating layer which is then cured, and then the second coating layer is applied and cured. In the simultaneous dual coating arrangement, both coats are applied after which they are cured.
During the coating process, when the fiber passes through the liquid prepolymer, the glass surface of the fiber pulls the meniscus of the liquid prepolymer into a cusp, which results in the entrainment of air bubbles in the coating. The bubble entrainment increases with draw rate. High speed drawing and coating is desirable for reducing cost of the fiber manufacturing operation, but draw speed must be tempered with preventing excess bubble formation in the coating. Bubbles give rise to a number of problems. Bubbles can cause losses in signal transmission by, for example, causing inhomogeneity of the modulus near the glass surface which in turn may cause mechanical distortion of the fiber. Bubbles can also weaken the mechanical strength of the coated fiber. Bubbles at or near the fiber surface interfere with inking and cause cosmetic concerns.
The fluid dynamics of bubble formation has been studied in detail. See e.g., S. F. Kistler, Hydrodynamics of Wetting, Wettability, ed. J. C. Berg, Marcel Dekker, New York, New York (1993). When the coating speed is less than the critical value at which air entrainment occurs, the contact line is generally normal to the motion of the substrate (e.g. the fiber). As the speed exceeds a critical value, the contact line appears to become serrated, or saw-tooth like, an observation that was first reported by T. D. Blake et al., Nature 282, pp.489-491 (1979). See also B. Bolton, et al., Chem. Eng. Sci. 35, pp. 597-601 (1980); M. T. Ghannam et al., AIChE J. 36, pp. 1283-1286. These authors have observed that bubble generation occurs at the leading cusps of the serrations, but the mechanism involved has not been investigated thoroughly.
In a recent study of free surface cusps, it was observed that powder on the surface of the coating fluid was swept into the interior of the fluid when the motion speed was sufficiently large. See J. -T. Jeong et al. J. Fluid Mech. 241, pp. 1-22 (1992). They did not observe bubble formation so that problem in a practical context was not addressed. Studies of two-dimensional cusps in a four roller apparatus (see G. I. Taylor, Proc. R. Soc. London, A146, pp. 501-523 (1934)) were reported by D. D. Joseph et al., J. Fluid Mech. 241, pp. 1-22 (1991). Other recent analytical work on cusped interface formation was reported by C. Pozrikidis, J. Fluid Mech. 357, pp. 29-57 (1998) and Y. D. Shiikhumurzaev, J. Fluid Mech. 359, pp. 313-328 (1998). The work reported in these references is relevant to the bubble formation problem since, in unpressurized coating applicators, the free surface becomes cusp-like from the shear introduced by the fiber motion.
The phenomenon that has been overlooked in studies of air entrapment is tip streaming. It occurs when a fine filament of air is enjected from the free surface cusp and breaks into minute bubbles via Rayleigh instability. It has been reported that when an interface becomes pointed, small bubbles can be ejected from the cusp. Such an event occurs only when the viscosity ratio of the two fluids is less than 0.1, a criteria which is satisfied by air and most coating fluids.
Although the phenomenon that leads to bubble formation in many coating operations, such as optical fiber coating, has been recognized and at least partly understood, the practical approach to preventing bubble formation in manufacturing technology is still largely empirical. For example, to avoid excessive bubble formation in optical fiber coating, the practice in the industry is to increase the draw rate until excessive bubbles appear, then back off the draw rate until a satisfactory coating is obtained. This empirical process, aside from being technologically impure, results in unnecessary scrap, and increases the attendance required of the draw tower operator. It also implies an unpredictability and uncertainty in the draw and coating process. This means that empirical establishment of draw speed must be repeated when coating conditions change, i.e. the pressure or the coating material changes, or when the applicator design changes.
There has been, and continues to be, increasing emphasis on optical fiber draw speeds. Much effort has been expended on increasing fiber velocity in the coating process while avoiding the formation of bubbles in the coating layers. In U.S. Pat. No. 4,246,299 of Ohls, a fiber is passed through an applicator having a die body that defines a small, vertically oriented, longitudinally tapered passage having a reservoir disposed about it. A series of radial ports provide fluid communication between the reservoir and the passage. Turbulence within the coating material, which causes entrapment of air bubbles, is reduced by maintaining the level of coating material in the passage. In U.S. Pat. No. 4,374,161 of Geyling et al. there is shown a coating arrangement wherein the fluid coating material is directed radially toward the fiber. The passage diameter for the fiber is large enough to prevent contact with the fiber, while the pressure of the fluid coating material is high enough to substantially prevent air from entering the applicator. In U.S. Pat. No. 4,480,898 of Taylor, there is shown a dual coating applicator having a die that provides for formation of a gap between the die and the first coated layer. A second die is located at the exit of the first die, with the second coating material flowing through a narrow passage between the first and second dies. The second die also provides for a gap so that the second layer is applied at a free surface at the point of contact with the first layer. This arrangement has been found to reduce instabilities and coating non-uniformities at increased speeds.
Despite these efforts to develop fiber drawing processes at high draw speeds, bubble formation with these processes is still unpredictable and they require, in general, constant monitoring and empirical adjustment of draw speed. It would be a major advance in the art if a technically sound method was available to predict the onset of bubble formation and relate it to draw speed so that proper draw speed can be used in the initial stages of fiber draw, and can be calibrated with changes in coating conditions.